Magnetron



Sept 1, 1953 GREENWALD 2,651,003

` MAGNETRON Filed July 1o, 1952 Illl #VVE/V701?, LEWIS GREENWALD,deceqsed #or-nes by sYLvIA GREENWALD, udmimsfmmx.

Patented Sept. 1, 1953 UNITED STATES PATENT OFFICE MAGNETRON v the ArmyApplication July 10, 1952, Serial No. 299,774

8 Claims. (Cl. 315-40) (Granted under Title 35, U. S. Code (1952),

sec. 266) The invention described herein may be manufactured and used byor for the Government for governmental purposes, without payment of anyroyalty thereon.

This invention relates to microwave oscillators and more particularly tointerdigital type magnetrons.

It is well known that although the pill-box cavity interdigitalmagnetron resonates at a single desired frequency, there are presenthigher modes of oscillation outside the desired resonant frequencyrange. Heretofore, interdigital magnetrons of this type have beenlimited to the cavity or zero-order mode of operation because in thismode the tooth structure presents an uncontaminated vr-mode fieldconfiguration to the interaction space, thus providing relatively highpower output and eiciency. When such interdigital magnetrons areoperated at frequencies in higher modes than the cavity mode, it hasbeen found that the power output and eiliciency is substantiallydecreased. The reason for this is probably that the interdigitalmagnetron structure does not present a field distribution to theelectrons which permits eilicient conversion to radio frequency power atthe higher modes.

It is therefore an object of this invention to provide an interdigitalmagnetron structure which will permit efficient operation at modeshigher than the cavity mode.

Another object of this invention is to provide an interdigital magnetronstructure which will permit 1r-mode operation for modes higher than thecavity or zero-order mode.

It is yet another object of this invention to provide a single cavityinterdigital magnetron having concentric interleaving sets of teeth foreiiciently utilizing the higher modes of oscillation.

In accordance with the present invention, the interdigital magnetronstructure includes a hollow cavity resonator terminated by a cylindricalwall and a pair of upper and lower spaced annular end plates. There isalso includes an axial cathode, a first set of interleaving teethcircumferentially disposed about said cathode comprising one series ofalternate teeth depending from the inner periphery of said upper annularplate and another series of alternate teeth depending from the innerperiphery of said lower annular plate. A second set of interleavingteeth cornprising two series of alternate teeth having discrete spacedsupporting means is circumferentially disposed within said first set ofinterleaving teeth and is concentrically arranged therefwith. Alsoincluded are means for coupling said second set of teeth to said firstset of teeth whereby the second set of interleaving teeth presents awmode distribution to the electron space charge at a preselected highermode of oscillation.

For a better understanding of the present in= vention, together withother and further objects thereof, reference is had to the followingdescription taken in connection with the accom panying drawing, inwhich:

Fig. 1 is a perspective view, partly in section, illustrating themagnetron structure embodied in the present invention; and

Fig. 2 is a transverse sectional View, taken on line 2-2 of Fig. 1, andillustrates the sinusoidal radio frequency field variation of a3rd-order mode of oscillation around the cavity anode.

Referring now to Figs. 1 and 2 of the draw-v ing, there is shown anelectron discharge device magnetron comprising an evacuated cavityresonator 3 having a cylindrical wall 5 terminated by spaced annularupper and lower end plates 4 and 6, respectively, which are axiallyaligned. A first set of uniformly spaced interleaving teeth l and Ill isconcentrically arranged about the axis of cavity resonator 3 at theinner peripheries of said annular end plates to form a cavity anode 8.Alternate cavity teeth lil extend downwardly from upper ring l2 which isaffixed to upper end plate Il. The other alternate cavity teeth I4extend upwardly from lower ring r6 which is afxed to lower end plate 6,so that, if one goes around the cavity anode in any one direction,alternate teeth are joined to the same annular end plate. Thus, cavityanode 8 comprises a plurality of oppositely placed sets of uniformlyspaced teeth extending toward each other from the inner periphery ofsaid annular end plates. Each of these teeth are of uniform length andwidth, the length of the teeth being approximately equal to the spacingbetween said annular end plates. A cathode is is axially and centrallypositioned within cavity resonator 3 and may be supported in the usualmanner. The usual magnetic and electric field between cavity anode 8 andaxial cathode I8 may be provided in the conventional manner to producean interaction electron space charge therebetween. Although twelve teethare shown, it is to be understood that the interdigital magnetron isessentially a single cavity resonator and that the number1 of modes isnot primarily dependent on the number of cavity teeth.

The interdigital cavity anode described hereinabove is well known in theart. The modes which may exist in such a cavity anode when excited by ahigh radio frequency field is described by Joseph F. Hull and Arthur W.Randalls in the Proceedings of the Institute of Radio Engineers,November 1948, pp. 1357-1363. The lowest order mode which may exist isknown as the cavity Inode or zero-order mode in which there is a 180phase difference between adjacent cavity anode teeth. It has beendetermined that the field and current distribution of the NIch ordermode have N sinusoidal variations around the circumference of the cavityanode, and, accordingly, the amplitudes of the radio frequency fieldsfor a preselected mode varies sinusoidally around cavity teeth 8. Thus,the voltage distribution between adjacent teeth will also varysinusoidally. For example, if the 3rd-order mode of operation isconsidered, the amplitude of the radio frequency fields and, hence, thevoltage distribution between adjacent teeth, will vary sinusoidallyaround cavity anode 8 making three complete cycles, This is clearlyshown in Fig. 2, wherein the solid line indicates the sinusoidalvariation of the radio frequency potential existing on alternate cavityteeth I8 and the dashed line indicates the sinusoidal variation of theradio frequency potential existing on the other alternate cavity teethi4. As shown in Fig. 2, there is a 180 phase relationship between thesinusoidal variations.

In accordance with the present invention, a second set of uniformlyspaced interleaving teeth 24 and 25 is concentrically arranged aboutcathode I8 and radially spaced from cavity anode [i to form an inneranode 20. Alternate teeth 22 of said inner anode extend downwardly fromLring 24 which is coplanar with upper ring l2. To other alternate inneranode teeth 25 extend upwardly from ring 28 is coplanar with ring i8.Thus, if one goes around the inner anode in any one direction, alternateteeth are joined to the same ring. Each of the inner anode teeth are ofuniform length and width, the length of the teeth being approximatelyequal to the spacing between rings 24 and 26. Cavity anode 8 is spacedfrom inner anode 20 and supported in position by means of symmetricallydisposed triangular shaped tabs 30, extendingr radially from rings 24and 26. These tabs couple the radio frequency fields excited along theinner anode to drive the cavity anode in the preselected mode. Inneranode 20 is so positioned with respect to cavity anode 8 that the apicesof the tabs 38 are attached to cavity anode 8 at points of equalpotential for the selected higher order mode of oscillation, preferablyat points of maximum amplitude potential. As shown, the tabsrespectively aiiixed to rings 24 and 26 are linearly aligned so thatthere is a 180 phase relation between oscillations of adjacent teeth ofinner anode 28. It is to be understood, of course, that the number oftabs extending radially from each of the rings 24 and 28 is a functionof the higher mode of operation. For example, if the 3rdorder modeoperation is desired, then three symmetrically disposed tabs will extendradially from ring 24 and connect to ring I2 and three similarlydisposed tabs will connect ring 28 to ring I6. Thus, for the 3rd-ordermode of operation the electrons in the interaction space move in fieldssimilar to those encountered in the usual qr-mode operation.

In operation, the electron space charge sweeping past the inner anodeteeth 20 induces voltages therebetween to excite high frequency elds inthe cavity anode. Since the inner anode presents a vr-mode potentialdistribution to the electron space charge for the preselected highermode of operation, the coupling tabs 3D will drive the cavity anode 8 inthe preselected mode of operation. Thus, one larger cavity may be usedto obtain higher frequencies by utilizing the higher modes ofoscillation of such a cavity, while at the same time using a magnetroninteraction space capable of exciting the cavity in an efficient manner.When the magnetron oscillates at the selected higher mode, the teeth Aofcavity anode 8 provide the necessary shielding between inner anode 28and the cavity resonator 3.

The addition of the inner anode 28 may add capacity to the cavity anode8 at points where 'the tabs 38 connect thereto. Ihis may be overcome byincreasing the spacing between ends of the interleaving cavity anodeteeth adjacent said tabs so that the capacity of the teeth of inneranode 20, when added to the capacity of the cavity anode teeth, restoresan even distribution of the capacity loading of cavity resonator 3.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to those`skilled in the art that various changes and modifications may be madetherein, without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit .and scope of the invention.

What is claimed is:

1. A hollow cavity resonator having an outer cylindrical wall connectedto upper and lower annular plates; a cathode axially and centrallypositioned within said resonator; a first set of interleaving teethcomprising one series of alternate teeth depending from the innerperiphery of said upper plate and a second series of alternate teethextending from the inner periphery of said lower plate; a second set ofinterleaving teeth circumferentially disposed within said first set ofteeth and concentric therewith, said second set of teeth comprising twoseries of alternate teeth, each alternate series of said second .set ofteeth having discrete supporting means; and means connecting said secondset of teeth to said first set of teeth whereby said second set ofinterleaving teeth presents a 1r-mode distribution to the electron spacecharge from said cathode at a preselected higher mode of oscillations.

2. The device set forth in claim 1 wherein said last mentioned meanscomprises a plurality of symmetrically disposed tabs extending radiallyfrom said supporting means, the number of said tabs extending radiallyfrom each of said supporting means being equal in number to saidpreselected higher mode of oscillation.

3. A hollow cavity resonator having a cylindrical wall; a cathodeaxially and centrally positioned within said resonator; a cavity anodecomprising two series of alternate teeth circumferentially disposedabout said cathode forming a first set of interleaving teeth, eachalternate series of teeth having discrete axially spaced outersupporting means affixed to said wall; an inner anode comprising twoseries of alternate teeth circumferentially disposed about said cathodeforming a second set of interleaving teeth, each alternate series ofsaid inner anode teeth having ldiscrete axiallyv spaced inner supportingmeans, said second set of teeth being within said rst set of teeth andconcentric therewith; a first and second group of symmetrically disposedtabs extending radially from said inner supporting means to said outersupporting means, said tabs being axially aligned and connected to saidouter supoprting means at points of equal potential at a preselectedhigher mode of oscillation.

4. A hollow cavity resonator comprising a cavity anode, said anodecomprising two series of alternate teeth circumferentially disposedforming a iirst set of interleaving teeth, each alternate series oiteeth having discrete axially spaced outer supporting means; an inneranode comprising two series of alternate teeth circumferentiallydisposed forming a second set of interleaving teeth, each alternateseries of said inner anode teeth having discrete axially spaced innersupporting means, said second set of teeth being within said first setof teeth and concentrically arranged therewith; a first group ofsymmetrically disposed tabs extending radially from one of said innersupporting means and connecting one alternate series of said rst set ofinterleaving teeth and one alternate series of said second set ofinterleaving teeth; and a second group of symmetrically disposed tabsextending radially from the other inner supporting means and connectingthe other alternate series of said first set of interleaving teeth tothe other alternate series of said second set of interleaving teeth; thetabs in said groups being linearly aligned and the number of tabs ineach group being equal in number to a preselected higher mode ofoscillation.

5. A hollow cavity resonator having an outer cylindrical wall connectedby upper and lower axially spaced annular plates; an outer anodecomprising two series of alternate teeth circumferentially disposed onthe inner periphery of said plates forming a rst set of interleavingteeth, one series of alternate teeth extending upwardly from said lowerplate and the other series of alternate teeth extending downwardly fromsaid upper plate; an inner anode comprising a pair of upper and loweraxially spaced supporting rings, two series of alternate teethcircumferentially disposed on said supporting rings forming a second setof interleaving teeth within said outer anode and concentricallyarranged therewith, one alternate series of said inner anode teethextending downwardly from said upper supporting ring, the otheralternate series of said inner anode teeth extending upwardly from saidlower supporting ring; a first group of symmetrically disposed tabsextending radially from said upper supporting ring to said upper annularplate; and a second group of symmetrically disposed tabs extendingradially from said lower supporting ring to said lower annular plate,said tabs being linearly aligned and the number of tabs in each of saidgroups being equal in number to a preselected higher mode ofoscillation.

6. The device set forth in claim 5 wherein said upper and lower annularplates are respectively coplanar with said upper and lower supportingrings.

7. A hollow cavity resonator having a cylindrical wall connected byupper and lower axially spaced annular plates; an outer anode comprisingtwo series of alternate teeth circumferentially disposed on the innerperipheries of said plates forming a rst set of interleaving teeth, oneseries of alternate teeth extending upwardly from said lower plate andthe other series of alternate teeth extending downwardly from said upperplate; an inner anode comprising a pair of upper and lower axiallyspaced supporting rings, two series of alternate teeth circumferentiallydisposed on said supporting means forming a second set of interleavingteeth within said outer anode and concentric therewith, one alternateseries of said inner anode teeth extending downwardly from said uppersupporting ring, the other alternate series of said inner anode teethextending upwardly from said lower supporting ring, a rst group ofsymmetrically disposed tabs extending radially from said uppersupporting ring to said upper annular plate; and a second group ofsymmetrically disposed tabs extending radially from said lowersupporting ring to said lower annular plate, said tabs being linearlyaligned and connected to points of equal potentials on said outer anode.

8. A hollow cavity resonator comprising a iirst set of circumferentiallydisposed interdigital teeth; a cathode axially and centrally positionedwithin said first set of teeth; a second set of circumferentiallydisposed interdigital teeth within said first set of teeth andconcentric therewith; and means extending radially from said second setof teeth and connecting to said rst set of teeth whereby said second setof teeth presents a fr-mode distribution to the electron space chargefrom said cathode at a preselected higher mode of oscillation.

SYLVIA GREENWALD, Administratrz'x of the estate of Lewis Greenwald,deceased.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,525,721A Posthumus Oct. 10, 1950 2,597,506 Ludi May 20, 1952

